1. Field of the Invention
The present invention relates to unleaded aviation gasoline of high motor octane number, low deposit formation, non-fouling and a freezing point of −58° C. or lower, to an additive concentrate and to the method for producing the additive concentrate.
2. Description of the Related Art
The high octane requirements of aviation gas for use in piston driven aircraft which operate under severe requirements, e.g., aircraft containing turbo-charged piston engines, require that commercial aviation fuels contain a high performance octane booster. The organic octane boosters for automobile gasolines (Mogas) such as benzene, toluene, xylene, methyl tertiary butyl ether, ethanol, and the like, are not capable by themselves of boosting the motor octane number (MON) to the 98 to 100+ MON levels required for aviation gasolines (Avgas). Tetraethyl lead (TEL) is therefore a necessary component in high octane Avgas as an octane booster.
Compositionally, Avgas is different from Mogas. Avgas, because of its higher octane and stability requirements, is typically a blend of isopentane, alkylate, toluene and tetraethyl lead. A typical Avgas base fuel without octane booster such as tetraethyl lead has a MON of 88 or higher, typically 88 to 97. Mogas, which has lower octane requirements, is a blend of many components such as butane, virgin and rerun naphtha, light, intermediate and heavy cat naphthas, reformate, isomerate, hydrocrackate, alkylate and ethers, or alcohols. Octane requirements of Mogas are based on research octane numbers (RON). For a given fuel, the RON is on average 10 octane numbers higher than its corresponding MON. Thus, the average premium Mogas possesses a MON of 86 to 88, whereas current Avgas must have a MON of 99.5. MON, not RON, is the accepted measure of octane for Avgas and is measured using ASTM D2700-92.
Conventional octane booster for Mogas, such as benzene, toluene, xylene, methyl tertiary butyl ether and ethanol are capable of boosting the MON of unleaded Avgas to the 92 to 95 MON range if added to Avgas in high enough concentrations. As noted previously, this is insufficient to meet the needs of 98 MON high octane Avgas.
With the phasing out of tetra-ethyl lead as an octane booster resort must be made to other means for boosting octane.
U.S. Pat. No. 5,470,358 teaches a high octane unleaded aviation gasoline comprising unleaded aviation gasoline base fuel having a motor octane number of 90-93 and an amount of at least one aromatic amine effective to boost the motor octane number of the base fuel to at least about 98, the aromatic amine having the formula
wherein R1 is C1-C10 alkyl, n is an integer of from zero to 3 with the proviso that R1 cannot occupy the 2- or 6-position on the aromatic rings.
Alternatively the fuel can comprise the same base fuel and an amount of at least one aromatic amine effective to boost the motor octane number of the base fuel to at least 98, said aromatic amine being a halogen substituted phenylamine or a mixed halogen and C1-C10 alkyl substituted phenylamine again with the proviso that the alkyl group cannot occupy the 2- or 6-position on the phenyl ring.
Preferred halogens are Cl or F. When R1 is alkyl, it occupies the 3-, 4-, or 5- (meta or para) positions on the benzene ring. Alkyl groups in the 2- or 6-position result in aromatic amines which cannot boost octane to a MON value of 98. Examples of preferred aromatic amines for octane improvement include phenylamine, 4-tert-butylphenylamine, 3-methylphenylamine, 3-ethylphenylamine, 4-methylphenylamine, 3,5-dimethylphenylamine, 3,4-dimethylphenylamine, 4-isopropylphenylamine, 2-fluorophenylamine, 3-fluorophenylamine, 4-fluorophenylamine, 2-chlorophenylamine, 3-chlorophenylamine and 4-chlorophenylamine. Especially preferred are 3,5-dimethylphenylamine, 3,4-dimethylphenylamine, 2-fluorophenylamine, 4-fluorophenylamine, 3-methylphenylamine, 3-ethylphenylamine, 4-ethylphenylamine, 4-isopropylphenylamine and 4-t-butylphenylamine.
U.S. Pat. No. 5,851,241 and its continuation U.S. Pat. No. 6,258,134 are directed to aviation fuel compositions which contain a combination of an alkyl tertiary butyl ether, an aromatic amine and optionally a manganese component such as methyl cyclopentadenyl manganese tricarbonyl (MMT). The base fuel to which the additive combination may be added may be a wide boiling range alkylate base fuel. According to the patents the combination of the alkyl tertiary butyl ether, the aromatic amine and, optionally, the manganese component result in a synergistic combination while boosts the MON of the fuel to a degree greater than the sum of the MON increases for each additive when used individually in the base fuel.
Heretofore, the aromatic amines which have been investigated, while exhibiting the ability to boost MON of aviation gasoline to 98 and higher have also been found to be susceptible to fouling and deposit formation and/or do not produce a fuel meeting the industry standard for freezing point of −58° C. or lower.
It is desirable to find a way to reduce the freezing point of aviation gasoline preferably unleaded aviation gasoline to −58° C. and lower, avoid deposit formation and be non-fouling in aviation gasoline of reduced toluene content while retaining high MON of at least 98.